A GaN-based nitride semiconductor has a wider band gap and higher electron mobility than those of Si or GaAs, and therefore, has been expected in application to a transistor for use in high breakdown voltage, high power and high frequency, and has been actively developed in recent years. Among such transistors, a transistor having a normally OFF property is useful, and a configuration for providing the normally OFF property has been examined.
For example, Patent Document 1 (International Publication WO/2010/064706) has disclosed a MIS-type field effect transistor using a group III nitride semiconductor layer, which is turned on by a practical positive gate voltage and can be operated at a high speed.
Moreover, Patent Document 2 (Japanese Patent Application Laid-Open Publication No. 2014-146744) has disclosed a high electron mobility transistor which has a mesa type cap layer (made of p-type GaN or AlGaN) provided with Schottky connection between the cap layer and the gate electrode.